Liquid crystal imprinting

ABSTRACT

Oriented materials and methods for their formation are disclosed. The oriented material is formed by depositing an oriented component from an oriented liquid crystal medium. Oriented materials having multiple layers and methods for their formation are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.09/641,450, filed Aug. 17, 2000, now U.S. Pat. No. 6,723,396 whichclaims the benefit of U.S. Provisional Application No. 60/149,391, filedAug. 17, 1999, each expressly incorporated herein by reference in itsentirety.

FIELD OF THE INVENTION

The present invention relates to a highly ordered material and, moreparticularly, to a highly ordered material obtained from a liquidcrystal medium.

BACKGROUND OF THE INVENTION

Preparation of new materials by directed synthesis at molecular-lengthscales is the object of much current research in materials science,surface chemistry, and the emerging field of crystal engineering. Aguiding principle of these efforts is the concept that rationalsynthesis using nested levels of structural hierarchy spanning a rangeof length scales from nanometers to the macroscopic could producesystems with tailored physical and chemical properties. The fundamentalpremise that underlies this approach is that the properties of amaterial are substantially determined by the length scales thatcharacterize its structure and organization. Thus, the mechanicalproperties of nanostructured composites, the electronic properties ofsemiconductor clusters, the magnetic properties of metallicsuperlattices, and the solution properties of colloidal suspensions allcorrelate directly to the nanometer-scale dimensions and structure thatcharacterize these systems. In the past ten years a number of newmethods based on nanosynthetic and “crystal engineering” strategies haveemerged. However, these methods remain limited both in the range ofdifferent types of building blocks that can be used, and in the extentto which molecular- and mesoscopic-scale order can be controlled. Novelapproaches are needed to extend synthetic control to include new typesof building blocks, for controlling molecular-scale order in thin filmsand crystals, and for controlling structure over macroscopiclength-scales.

Liquid crystal (LC) solvents offer several advantages over conventionalliquid media for solution phase synthesis of solid materials. Theseadvantages stem from three characteristics unique to LC fluids: (1) LCsundergo strong directional coupling to solid surfaces; (2) LCs possessanisotropic (direction-dependent) properties, including varioustransport, optical, and mechanical properties; and (3) long-rangeorientational order in a LC fluid can be manipulated using an externalfield. Each of these characteristics can be exploited in different waysto control the structure and organization of a material prepared usingLC growth media. The mechanism for controlling order in a given systemdepends on the type of building block and the pathway to building blockaggregation.

A major challenge in the emerging fields of nanostructured materialssynthesis and crystal engineering is to devise general fabricationmethodologies applicable to a diverse set of fundamental building blocksand capable of producing assemblies in which structure and organizationare controlled over a broad range of length scales. Interest incontrolled crystallization stems from recognition that many macroscopicchemical and physical properties are determined by the microscopicarrangement of a material's basic chemical components, and by the needto prepare well ordered aggregates from building blocks that do notreadily crystallize, such as some proteins. Current approaches relyeither on specific intermolecular interactions to produce spontaneousself-organization, anisotropic interactions between the building blockand an external field, or on a template of seed crystals or a lyotropicliquid crystal. Most methods suffer from rather severe chemical andphysical constraints on the choice of fundamental building block, andthe size scale characterizing structure and organization is only partlycontrolled.

The use of LCs as solvents for controlled crystallization and materialssynthesis has not been widely studied. A method for controllingmolecular alignment in an organic film through the use of a LC has beenreported. See U.S. Pat. No. 5,468,519, entitled “Method For Forming anOrientation Film Including Coupling an Organic Compound to a SilaneCoupling Agent in a Magnetic or Electrical Field”. This patent statesthat films formed by the method would be useful anchoring layers inLC-based optoelectronic devices.

Polymers are the only major class of materials at have been studied withthermotropic LCs. Some polymers, such as KEVLAR and spider silk, arethought to pass through a LC phase while curing. The resulting frameworkof partially oriented chains imparts various desirable properties.Partly for this reason a variety of methods have been developed toincorporate LC behavior into polymers. The most important class of thesesystems is liquid crystal polymers (LCP), which are synthetic polymersconsisting of a flexible backbone to which small LC monomers areperiodically attached. The monomers may be calamitic or discotic, andmay be attached to the backbone by a linker or may be incorporated intothe backbone itself. LCPs have been studied as melts, and as solutionsin LC solvents. Oriented polymer materials may be formed by curing froma LC phase. Macroscopic alignment can be achieved by poling with anexternal alignment field. Polymer-stabilized LCs are a related systemconsisting of an open polymer framework filled by a LC fluid. Thesesystems are being explored for use in LC display devices because theyprovide high optical contrast and they are relatively insensitive tomechanical stress and domain formation. During manufacture,polymerization to form the framework is carried out using a LC solventin an external alignment field, resulting in partial alignment of thepolymer precursors. After curing memory of the original alignment isretained by the composite. Chiral nematic solvents have also been used,although less commonly. A recent example is the polymerization ofacetylene in a chiral nematic environment that resulted in helicalstrands whose handedness (clockwise or counterclockwise) was determinedby the LC.

Several existing nanosynthetic approaches involve lyotropic liquidcrystals. In one method, referred to as LC-templating (LCT), a tropic LCprovides an organized scaffolding promoting condensation of an inorganicbuilding block to form a (three-dimensional) ceramic-like framework.Inorganic precursors remain confined to the aqueous environment of thesurfactant/water mixture and interact with the polar surfactantheadgroups through coulombic or hydrogen-bonding forces. Aftercondensation the organic framework may be removed, leaving a mesoporousmaterial whose structure, pore size, and symmetry are determined by theLC scaffolding. This general approach has been creatively applied toproduce several new types of nanostructured inorganic material, the mostnotable example perhaps being the synthesis of the M41S family ofmesoporous sieves. LCT has also been used to prepare nanostructuredmetal clusters and patterned metallic films. These syntheses almostalways result in polycrystalline materials with small grain sizes (˜μmscale).

There is growing interest in the fabrication of highly ordered molecularfilms for a range of applications, and considerable effort has beeninvested in the molecular design, synthesis, and characterization ofcrystalline films with targeted properties. However, a major limitationto constructing useful devices based on molecular materials, and toobtaining a better understanding of the properties of molecular solids,is that most organic compounds of interest yield polycrystalline filmswith random or partially random domain orientation. Numerousapplications, ranging from molecular electronics and photonics toprotein crystallography would benefit from a general method for growingfilms with uniform alignment.

Despite the advances noted above, there remains a need for highlyordered materials that can be readily formed. A need also exists for amethod for readily forming a highly ordered material. The presentinvention seeks to fulfill these needs and provides further relatedadvantages.

SUMMARY OF THE INVENTION

The present invention provides highly ordered materials and methods fortheir formation. The method is compatible with a wide variety offundamental building blocks and can therefore provide a variety ofhighly ordered materials including molecular films. The method providesoriented materials that can have uniform structure and organizationspanning from molecular to macroscopic dimensions. The method includesthe use of an oriented liquid crystal medium from which is deposited anoriented component to provide an oriented material.

In one aspect, the invention provides a method for forming an orientedmaterial. In the method, a component is deposited from an orientedliquid crystal medium containing the component to provide the orientedmaterial. In the oriented medium, the component is orientationallyordered, which results in the deposited component being oriented. In oneembodiment, the oriented material is formed using a thermotropic liquidcrystal solvent, which provides an anisotropic medium that cancommunicate the influence of an external alignment field to a componentin the liquid crystal medium. The oriented component is ultimatelydeposited from the oriented, component-containing liquid crystal mediumto provide the oriented material. The liquid crystal solvent allows fororientational control over the solute, even if the solute does notinteract directly with the field.

In another aspect of the invention, oriented materials are provided. Inone embodiment, the oriented material is a film. The film can be amonolayer film or, alternatively, the film can include more than onelayer. For embodiments that include more than one layer, each layer canbe selectively oriented. For multilayered oriented materials, each layercan also include different components.

In further aspects, the invention provides methods for using and devicesthat include the oriented materials formed in accordance with thepresent invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of thisinvention will become more readily appreciated by reference to thefollowing detailed description, when taken in conjunction with theaccompanying drawings, wherein:

FIG. 1 is a schematic illustration of liquid crystal imprinting;

FIG. 2 is a schematic illustration of the effect of a magnetic field onnuclei and anchoring;

FIG. 3 is a schematic illustration of a method for forming an orientedmaterial;

FIG. 4 is a scanning tunneling microscopic (STM) image (111×111 nm) ofcrystalline rows of 4′-octyl-4-cyanobiphenyl (8CB) on a graphitesubstrate prepared in the absence of a magnetic field, domainorientation is random;

FIGS. 5A and 5B are STM images (9.5×8.0 nm) of a crystalline monolayerof 8CB on graphite with right (R)- and left (L)-handed domains formed athigh field strength;

FIGS. 5C-5E are histograms illustrating 8CB distribution in monolayerson graphite, normalized probability is plotted versus row-field angle(φ) in degrees for applied field strengths of 0.7 T, 0.1 T, and nofield, respectively, the solid line in 5C is the predicted distribution;

FIG. 6 is a STM image (10×10 nm) illustrating molecular alignment in amonolayer film of tetracosanoic acid (TA) on graphite, the arrowindicates the direction of the external field;

FIG. 7 is a histogram illustrating TA distribution in a film depositedon highly oriented pyrolytic graphite (HOPG) from LC ZLI-1565,normalized observation probability is plotted versus molecularaxis-field angle (φ) in degrees, the solid line is a guide to the eyewith a fitted centroid at 12±5°;

FIG. 8 is a STM image (90×88 nm) an 8CB/n-tetracontane domain boundaryin a mixed film, 8CB is on the right;

FIG. 9 is a representative hybrid LC-based magneto-optical phase-changememory device; and

FIG. 10 illustrates compounds useful in rewriteable anchoring films.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides highly ordered materials and methods fortheir formation. The method is compatible with a wide variety offundamental building blocks and can therefore provide a variety ofhighly ordered materials including molecular films. The method providesoriented materials that can have uniform structure and organizationspanning from molecular to macroscopic dimensions. The method includesthe use of an oriented liquid crystal medium from which is deposited anoriented component to provide an oriented material.

As used herein, the terms “highly ordered”, “orientationally ordered”and “oriented” are used interchangeably and refer to a material in whichat least one of its constituent components exhibit substantial uniaxialalignment.

In one aspect, the invention provides a method for forming an orientedmaterial. In the method, a component is deposited from an orientedliquid crystal medium containing the component to provide the orientedmaterial. In the oriented medium, the component is orientationallyordered. This ordering in the liquid crystal medium results in thecomponent being deposited on a substrate maintaining the orientationalorder of the medium.

The liquid crystal medium can be oriented by any external influencecausing alignment of the LC medium. Representative methods include amagnetic field, an electric field, the use of an alignment layer on asurface in contact with the LC medium, shearing the LC medium, orcausing the medium to flow.

In one embodiment, the oriented material is formed using a thermotropicliquid crystal solvent, which provides an anisotropic medium that cancommunicate the influence of an external alignment field to a component(e.g., small organic molecule) in the liquid crystal medium. Theoriented component is ultimately deposited (e.g., as a film) from theoriented, component-containing liquid crystal medium to provide theoriented material. The liquid crystal solvent allows for orientationalcontrol over the solute (i.e., building block), even if the solute doesnot interact directly with the field.

Suitable liquid crystal solvents include pure materials, such as4′-octyl-4-cyanobiphenyl and 4-methoxybenzylidene-4-butylaniline, aswell as LC mixtures, such as ZLI-1565 and ZLI-2116 (commerciallyavailable from Merck).

Suitable components or building blocks include small organic moleculesthat can act as solutes in the liquid crystal solvent. Other suitablecomponents include macromolecular structures including polymers.Suitable components also include particles having sizes in the rangesfrom nanometers to microns. Representative particles include carbonnanotubes and other fullerenes; fibers and whiskers composed ofmaterials such as SiC and carbon; inorganic particles, such assemiconductor and metallic clusters; microtubules; viral particles;nucleic acid polymers (DNA); and organic particles, such as polystyrenespheres.

The liquid crystal medium (i.e., the liquid crystal and component to bedeposited) need not be a true solution. The liquid crystal medium can bea mixture or suspension of a liquid crystal and a component to bedeposited. The liquid crystal medium need only be capable of providing amedium in which the component to be deposited can be orientationallyoriented. The liquid crystal medium can include more than one liquidcrystal. The liquid crystal medium can include more than one componentto be deposited.

In addition to a method for preparing an oriented material having asingle oriented layer, the present invention provides methods forforming materials having more than one layer (i.e., multiple layers)with at least one of the layers being oriented. For materials thatinclude more than one layer, additional layers are formed by thesequential deposition of a component onto the surface of the previouslydeposited layer. The additional layers can be oriented as describedherein or, alternatively, can be randomly oriented.

In general, the invention provides a method for forming an orientedmaterial. The method includes obtaining a liquid crystal medium thatincludes comprising a liquid crystal solvent and an orientationallyorderable component; contacting the liquid crystal medium with a surfacefor receiving the orderable component; applying an influence to theliquid crystal medium to provide an oriented liquid crystal medium, withthe orderable component being orientationally ordered in the orientedmedium; and depositing the orientationally ordered component onto thesurface to provide the oriented material.

In one embodiment, the invention provides a method for forming anoriented material in which an oriented component is deposited from anoriented liquid crystal medium onto a substrate. As used herein, theterm “substrate” refers to a material having a surface for receiving acomponent deposited from a medium, such as an oriented componentdeposited from a liquid crystal medium. The substrate can have a varietysurfaces. For example, the substrate can have a surface that does notinclude a deposited material (e.g., a graphite, glass, metal, or plasticsurface). Alternatively, the substrate can have a surface that doesinclude one or more layers of materials, including deposited materials.

In others embodiments, the invention provides methods for forming anoriented material having at least two layers. In one method, a firstoriented component is deposited from a first oriented liquid crystalmedium onto a substrate to provide a first oriented layer followed bydepositing a second oriented component from a second oriented liquidcrystal medium onto the first oriented layer to provide a secondoriented layer adsorbed onto the first layer. In another embodiment, anoriented component is deposited from an oriented liquid crystal mediumonto a substrate to provide an oriented layer followed by depositing asecond component from a liquid crystal medium onto the oriented layer toprovide a second layer adsorbed onto the first layer. The second layercan include materials that are randomly oriented. In a furtherembodiment, a first component is deposited from a liquid crystal mediumonto a substrate to provide a first layer followed by depositing anoriented component from an oriented liquid crystal medium onto the firstlayer to provide an oriented layer adsorbed onto the first layer. Thefirst layer can include materials that are randomly oriented.

In another embodiment, the invention provides a method for forming amaterial having a plurality of layers in which at least one of thelayers is an oriented layer. In the method, a first component isdeposited from a first liquid crystal medium onto a substrate to providea first layer followed by sequentially depositing successive componentsfrom successive liquid crystal media to provide successive layers. Eachsuccessive layer is adsorbed onto the previously deposited layer. Inthis embodiment, at least one layer is an oriented layer formed bydepositing an oriented component from an oriented liquid crystal medium.

It will be appreciated that multilayered materials having one or moreoriented layers includes materials in which the oriented layer includesan oriented component having a selected but variable orientationthroughout the thickness of the oriented layer. Such a layer can beprepared by varying the orientation of a liquid crystal medium duringthe deposition of an oriented component contained within the medium. Forexample, by rotating the orientation of the liquid crystal medium (by,for example, rotating the applied magnetic field) an oriented layer canbe formed in which the oriented component in the layer has varyingorientations (e.g., a helical configuration). It will be appreciatedthat other orientation configurations can be readily obtained by varyingthe orientation of the liquid crystal medium from which a component isdeposited and that all such configurations are within the scope of theinvention.

In another aspect of the invention, oriented materials are provided. Inone embodiment, the oriented material is a film. Such a film can be amacroscopically uniaxial molecular film. The film can be a monolayerfilm. Alternatively, the film can include more than one layer. Forembodiments that include more than one layer, each layer can beselectively oriented. For multilayered oriented materials, each layercan also include different components. Materials having, in addition toan oriented layer, a randomly oriented layer are also provided by theinvention.

In one embodiment, the present invention provides a thin film in whichorientational order is controlled. In one embodiment, the thin film is amonolayer film. The film is formed using building blocks of smallorganic molecules soluble in a thermotropic LC solvent. When a quantityof a molecular solute (i.e., the building block) is dissolved in the LC,nematic order can be imprinted on the monolayer deposited onto a solidsubstrate. In the presence of an external influence causing alignment ofthe LC medium, the monolayer film develops orientational order overmolecular to macroscopic length scales.

In one embodiment, the invention provides a material having at least oneoriented layer in which the oriented layer is formed by depositing anoriented component from an oriented liquid crystal medium.

In another embodiment, the invention provides a multilayered materialhaving at least one oriented layer in which the oriented layer or layersare formed by depositing an oriented component from an oriented liquidcrystal medium.

The above process is referred to as liquid crystal imprinting (LCI). TheLCI process is illustrated schematically in FIG. 1. Referring to FIG. 1,a solution (or mixture) containing a solute in a LC solvent is prepared(A) and applied to a substrate in the presence of an external field (B).Solute molecules deposited from the mixture form a film in whichmolecular order is controlled by the orientation of the field anddeposition in the absence of an external field (left-hand side) resultsin a film with macroscopically random orientation (C).

LCI alignment is based on LC anchoring interactions. In a film formingat a LC interface, these interactions exert a small torque on nucleiwhose orientation differs from the orientation of the interfacialnematic director. If the director is fixed by an external field, nucleireorient to align themselves parallel to it. The origin of this torqueis illustrated in FIG. 2. Referring to FIG. 2, if the anchoringdirection a above a nucleus differs from the bulk director establishedby the field m, a twist distortion occurs at the interface, increasingthe system's orientational energy. By rotating through an angle θ thenucleus minimizes this energy. At most interfaces LCs tend to adopt apreferred orientation-the anchoring direction. If the anchoringdirection above a nucleus differs from the director orientation in thebulk imposed by the external influence causing alignment of the LCmedium, strain energy is stored in the fluid as a distortion in thedirector, and a quasi-elastic torque is exerted on the nucleus. Forplanar anchoring, a misaligned nucleus leads to a twist distortion, asshown in FIG. 2. A nucleus developing under these conditions experiencesan energetic incentive to orient in a manner that minimizes strainenergy by rotating to match the nucleus' anchoring direction with thatimposed by the field.

For nuclei forming on isotropic substrates; such as at an air/LCinterface or at a liquid/LC interface, an alignment energy as small as˜1 kT would be sufficient to produce films of highly uniformorientation. However for nuclei forming on anisotropic substrates, suchas on the surface of a crystal, the alignment energy will normally bemuch smaller than adsorbate-substrate interactions. In this case therole of LC anchoring is to remove or reduce any substrate orientationaldegeneracy, causing nuclei to select the substrate lattice directionnearest the direction that minimizes the strain energy.

For example, one system examined consisted of a monolayer film of thefatty acid C₂₃H₄₇COOH (tetracosanoic acid, TA) deposited from athermotropic LC solvent onto graphite. Tetracosanoic acid forms acommensurate, polycrystalline film with the molecular axis lying in thegraphite plane oriented along one of the principle graphite latticevectors. Although molecule-substrate interactions are very strong andanisotropic in this system, it was nevertheless possible to produceuniformly oriented monolayers when films were deposited in an externalinfluence causing alignment of the LC medium. Anchoring interactionseffectively eliminated the three-fold orientational degeneracy of thesubstrate by making it much more favorable for nuclei to orient alongthe graphite lattice direction most nearly parallel to the magneticfield.

An important feature of LCI is that the external field need not exertany direct influence on the building block at all; the field aligns theLC solvent, which through anchoring aligns the growing film. The onlyrequirement is that nuclei exhibit planar, nonhomogeneous anchoring.This requirement is easily satisfied for most building blocks by asuitable choice of the LC solvent. As a consequence, LCI is compatiblewith a wide range of building blocks. Suitable building blocks includeany molecule or particle that can undergo orientational ordering in aliquid crystal medium. Representative building blocks include smallmolecules; macromolecules; and particles, such as colloidal particles,carbon nanotubes and other fullerenes, fibers and whiskers composed ofmaterials such as SiC and carbon, inorganic particles such assemiconductor and metallic clusters, microtubules, viral particles,nucleic acid polymers (DNA), and organic particles such as polystyrenespheres, among others.

There are more than 70,000 thermotropic LC compounds known, spanning awide range of chemical properties and transition temperatures. This isimportant because the structure of crystals grown from solution usuallydepends upon the chemical characteristics of the solvent. Rationalselection of the solvent's chemical properties therefore lendsadditional control over crystal packing, defect density, inclusions andother factors.

The preparation of a representative oriented material, an oriented film,is illustrated schematically in FIG. 1. Results from threerepresentative systems are summarized in Table 1. The first entry inTable 1 represents the simplest implementation of the LCI method, inwhich the LC solvent and solute building block were identical.

TABLE 1 Representative LCI Systems. LC solvent Solute Outcome Comment4′-octyl-4- 8CB Uniformly aligned See FIG. 5. cyanobiphenyl monolayer(8CB) ZLI-1565 n-tetracosanoic Uniformly aligned See FIGS. 6 acidmonolayer and 7. 4′-pentyl-4- n-tetracosanoic Randomly aligned Themonolayer cyanobiphenyl acid monolayer crystallizes above (5CB) T_(N→1)when the LC is isotropic, and thus was ran- domly oriented.

Representative monolayer organic films were prepared in accordance withthe present invention by depositing solutes onto graphite substratesfrom different LC solvent/molecular solute combinations using bothnematic and smectic LCs. Two representative systems include: (1)n-tetracosanoic acid (TA) deposited from the nematic LC ZLI-1565(commercially available from Merck KGaA, Darmstadt, Germany) (0.3% byweight); and (2) 4′-octyl-4-cyanobiphenyl (8CB) deposited from a neatfluid. 8CB is a room-temperature smectic-A LC. The second systemrepresents the simplest implementation of the method, in which the LCsolvent and solute were identical. Films of TA and 8CB were depositedonto substrates of highly oriented pyrolytic graphite (HOPG ZYH grade,Advanced Ceramics, Inc.) measuring ˜1 cm , which were deeply immersed(>2 mm) in a reservoir of LC/solute mixture. Each system was heated to˜100 C, then gradually cooled to room temperature in a magnetic fieldoriented parallel to the substrate plane (see FIG. 3). In both systems,a single polycrystalline monolayer formed at the graphite interface incontact with the bulk LC solvent. After cooling, samples were removedfrom the field and analyzed with scanning tunneling microscopy (STM).The STM tip penetrated through the LC fluid to image molecules in themonolayer at the graphite interface.

Both 8CB and TA adsorb strongly on graphite, forming commensuratedomains oriented along one of three symmetric directions separated by120°. In the absence of an external influence causing alignment of theLC medium, these three directions are energetically equivalent. Becausethe crystallographic orientation of the substrate varied over micronlength scales, samples prepared with no external influence causingalignment of the LC medium developed macroscopically random orientation.8CB forms an epitaxial, polycrystalline monolayer on graphite. A STMimage showing crystalline rows of 8CB on a graphite substrate isillustrated in FIG. 4. Individual domains orient in a discrete set ofsix energetically-equivalent directions determined by the symmetry ofthe substrate and registry of the molecules. However, this symmetry wasbroken for films deposited within a magnetic field, leading tomacroscopically uniaxial order. To measure this uniaxial order, theorientation of adsorbed molecules was assessed at approximately 200widely spaced locations across several samples using STM.

FIG. 7 presents a histogram showing the experimental distribution(shaded bars) of TA alignment measured for several films prepared in a12.6 kG field. These films displayed macroscopically uniaxial ordercontrolled by the external field-nematic order has been imprinted on themonolayer. The mean angle between the long molecular axis and the fieldwas φ_(o)=12±5; the STM image (FIG. 6) shows the most probable domainorientation. To normalize for micron-scale variations in local substrateorientation, we plot P(φ)=N(φ)/[N(φ)+N(φ+120)+N(φ−120], where N(φ) isthe number of domains in which the long molecular axis oriented at anangle N with respect to the field. This procedure corrects forstatistical undersampling associated with making measurements at afinite number of surface locations and also eliminates any systematicorientational bias which would occur if the distribution of localsubstrate orientations included in the sampling was not truly random.

In 8CB monolayers, the long molecular axis also oriented approximatelyparallel to the field. However, in this case the distribution isbimodal, because we have plotted the angle between the molecular rowsand the field, rather than the angle involving the molecular axis (seeFIGS. 5A and 5B). For 8CB, row orientation is a more convenientdescriptor of overall alignment than the orientation of individualmolecules, since each molecular axis in the unit cell points in aslightly different direction, and because the 8CB monolayer is chiral.Chirality develops as a result of molecular adsorption, when rotationabout the C—C bond linking the cyanobiphenyl headgroup to the alkyl tailgroup is quenched. This gives the molecule a bow shape, and hencechirality in two dimensions. STM images of left- and right-chiraldomains are shown in FIGS. 5A and 5B. Because the molecular axis formsan angle of approximately ±30° to the rows, and because the sign of thisangle depends on domain chirality, there were two favored roworientations, φ_(o)=±(54.6±8.6°) with respect to the field. When rowsoriented in these directions, the molecular axis was essentiallyparallel to the field.

FIG. 5C shows the experimental distribution of 8CB row alignment in a7.2 kG field (0.7 T). Films prepared at other field strengths from 1.2to 13 kG resulted in an improvement in the quality of orientationalorder with increasing field strength, reaching a limit above ˜2 kG.FIGS. 5D and 5E show the experimental distribution of 8CB row alignmentat 0.1 T and no field, respectively. Alignment showed little additionalimprovement above this threshold, because adsorbates almost always chosethe local substrate vector making the row-field angle as close aspossible to ±54.6°, while maintaining substrate registry. Films preparedin the absence of a field showed random macroscopic orientational order.See, e.g., FIG. 4.

The 8CB monolayer began to crystallize at 27-29° C., after the bulk LChad cooled to the smectic-A phase. The TA monolayer crystallized at30-40° C., near the center of ZLI-1565's nematic range, based on thebehavior of related aliphatic adsorbates on graphite. Because thesubstrates were immersed in LC solutions at ˜100° C., moleculesinitially adsorbed in a thermally disordered state (i.e., they did notadsorb prealigned). Furthermore, the orientation of an aligned filmcould not be altered by placing it back in the field at a differentorientation, unless the temperature was raised above the monolayermelting point. Thus, molecular alignment originated after adsorption,but during an early stage of film growth, when molecules were formingsmall crystalline aggregates. Once the number of molecules in anaggregate grew beyond a critical value N, aggregate rotation was nolonger possible and orientation was fixed.

From these observations, a simple model can be constructed forcommunication of orientational order to the developing film. There arethree factors influencing adsorbate orientation: (1) interaction withthe substrate, (2) direct interaction with the external field, and (3)interaction with the LC fluid. The orientational energy of an adsorbatecan be written as a sum of these contributions,E _(orientation)(φ)=E _(substrate) +E _(field) +E _(fluid).

The first term represents adsorbate-substrate interactions, which haveby far the strongest influence. At the maximum field strength producedby a magnet (13 kG), molecules in both films remained commensurate withthe substrate, and the detailed structure of the unit cell was identicalto samples prepared with no field. The magnetic field's influence wastherefore limited to breaking the threefold orientational degeneracy ofthe substrate, causing domains to select the local substrate directionthat allowed molecules to most nearly align in the preferred direction.

A calculation of the second term, direct interaction of adsorbedmolecules with the external field, showed that it was insignificant.8CB's biphenyl moiety results in stronger anisotropic interactionswith-a magnetic field than TA, but-these interactions are nonethelessvery weak. The energetic cost of misaligning an entire domain of mmolecules by 90° in a field of strength H is E_(field)=mwΔ_(χ) H²/2,where Δ_(χ) is the mass magnetic susceptibility anisotropy and w is themolecular weight. The grade of HOPG used in this work resulted in anaverage adlayer domain size ˜1 μm², or m˜10⁶ molecules, so for 8CB,E_(field≦)10⁻² kT per domain in a 7.2 kG field at 28° C. This energy isat least 2 orders of magnitude too small to account for the statisticalexcess of favored orientations in FIGS. 5-7.

It is the third factor-interaction between aggregates of adsorbedmolecules and the LC fluid above them-that produces alignment in thesefilms. While surface-induced ordering of LCs is a well-known andtechnologically important phenomenon, the reverse process, i.e.,LC-induced surface ordering, has not to our knowledge been previouslydescribed. As is the case for surface-induced bulk alignment, adlayeralignment arises from the LC's anisotropic surface tension, which causesthe director to adopt a preferred orientation (“anchor”) at aninterface. In representative systems, the LCs underwent planaranchoring, meaning the director oriented parallel to the surface, alonga preferred azimuthal direction. If an aggregate's easy axis does notcoincide with the bulk director orientation imposed by the field, thefree energy of the system (aggregate+interfacial fluid) increases by anamount proportional to this mismatch. Although the field strengths usedhere had little direct influence on monolayer alignment, they weresufficient to align the bulk solvent, where macroscopic numbers ofmolecules act collectively. This was confirmed this using polarizingoptical microscopy. The following treatment of these anisotropicfluid-adlayer interactions qualitatively accounts for the observedalignment phenomena.

In a nematic solvent, excess free energy arises primarily from a twistdistortion in the director field above misaligned aggregates. Bytwisting, the LC balances the alignment torque exerted by the field withrestoring torque from curvature elasticity and surface anchoring. Thetwist distortion extends a distance d=H⁻¹(K₂₂Δ_(χ))^(1/2)˜1-10 μm intothe bulk, after which the director orients parallel to the field. HereK₂₂ is the twist elastic constant. The interdomain orientationalinteractions in 8CB films mediated by an interfacial LC fluid extendless than 0.25 μm laterally, which is less than the average terracesize. Thus, only isolated aggregates are considered. In the limit ofstrong anchoring (anchoring energy>>H(K₂₂Δ_(χ))^(1/2)˜10⁻⁶ Jm⁻²), theelastic torque exerted on each aggregate by the solvent has aparticularly simple form: τ=—NaH(K₂₂Δ_(χ))^(1/2) sin(φ₀−φ), where a isthe area per molecule in the crystalline film. If the condition forstrong anchoring is not satisfied, the director rotates at theinterface, somewhat reducing the torque exerted on the aggregate. Thefree energy cost of aggregate misalignment is found by integrating thetorque through the misalignment angle:E_(nematic fluid)=—NaH(K₂₂Δ_(χ))^(1/2) cos(φ₀−φ), where for TA,φ_(o)=12°.

In a smectic solvent, twist distortions are formally disallowed due tointerlayer incompressibility. Therefore in the limit of high fieldstrength, the director remains approximately parallel to the externalfield right down to the surface. In this case, the excess free energy ofaggregate misalignment arises from a mismatch between the directororientation at the surface and the aggregate's anchoring direction. Thisenergy is often modeled with the Rapini-Papoular potential [14]:E_(smectic fluid)=NaW/2 sin² (φ₀−φ), where W is the anchoring energy,and for 8CB, φ_(o)=±54.6°.

Using these expressions for E_(fluid), the orientational distribution ofan ensemble of aggregates can be calculated assuming the film developsin thermal equilibrium. The probability P(φ)=Q⁻¹ exp[−E_(fluid)(φ)/kT],where T is the temperature of monolayer formation, the partitionfunction Q=Σ_(φ) exp[−E_(fluid)(φ)/kT], k is Boltzmann's constant, and φis restricted to orientations consistent with local substrate registrydue to overwhelming adsorbate-substrate interactions.

These expressions were used to separately fit the distributions in FIGS.5 and 7 to find the aggregate size N that described each system best.For the anchoring energy of 8CB, W=5×10⁻⁵ J m⁻², was chosen as a typicalvalue on crystalline surfaces. The fitting procedure yieldedN_(TA)=2±2×10³ and N_(8CB)=6−7×10² molecules, with the calculateddistributions shown as solid lines in FIGS. 5C and 7.

The single-molecule alignment energy E_(fluid)/N<<kT at thecrystallization temperature. Therefore, only adsorbates that are membersof a crystalline aggregate—and hence act collectively—can possesssignificant uniaxial alignment. The aggregate size N is much smallerthan the average domain size m in the fully developed film, the latterbeing of the order m≅10⁶ molecules. This is consistent with theobservation that film orientation cannot be altered by replacing analigned sample in the field at a different orientation; the aggregateshave grown too large to rotate. Rotation presents an energetic barrierthat increases rapidly with aggregate size, because it entails atransition through an incommensurate state. The orientational statisticsmeasured from fully developed films thus reflect the alignmentdistribution during the early stages of film formation, because largeaggregates cannot overcome the rotational activation barrier onexperimental time scales.

Even in the strongest field, the maximum energy cost of aggregatemisalignment is only a small fraction of the thermodynamic driving forcefor nucleation. With 8CB, for example, the ratio of these two energieswas calculated to be about 0.1% at 28° C. This explains why the detailedarrangement of molecules within the unit cell is unchanged from filmsprepared outside the field. The field selects among degenerate aggregateorientations, but does not significantly perturb adsorbate-substrate oradsorbate-adsorbate interactions.

In a further aspect, the invention provides methods for using anddevices that include the oriented materials formed in accordance withthe present invention.

Composite Films with Selective Alignment. Representative composite filmsof the invention include bicomponent thin films in which one componentis uniformly aligned while the other is randomly (or partially)oriented. Composite materials consisting of a mixture of ordered anddisordered phases can exhibit useful combinations of physical, optical,and other properties. For example, the combination of hardness andtoughness possessed by materials such as tooth enamel and mollusk shellsresults from combining oriented inorganic microcrystals with adisordered biopolymer matrix. The degree of LC-induced alignment in thinfilms can be remarkably sensitive to the chemical nature of the buildingblock. For example, films of the alkane n-tetracontane (C₄₀H₈₂) resistalignment in most LCs. However, by replacing one terminal methyl groupwith a more polar moiety such as a carboxylic acid, unidirectionalalignment is readily achieved.

The present invention further provides composites (i.e., multicomponentfilms) in which the orientational order of each component isindividually controlled. One potential application of this kind ofmulti-component, selectively oriented film is as a template onto whichadditional material could be epitaxially deposited to produce athree-dimensional composite with tailored microstructural order.

Representative systems include the binary mixture 8CB/n-tetracontane andthe ternary mixture ZLI-1565/tetracosanoic acid/n-tetracontane. Thefirst of these two systems forms monolayers with mixed composition whenthe mole fraction of the alkane is ˜1/600 (FIG. 8). Variations inconcentration around this value change the monolayer composition.Although 8CB can be aligned, tetracontane deposited from 8CB is notaligned. In the second system, both building blocks-differ from the LCsolvent itself. In pure films grown from ZLI-1565, the fatty acid isaligned, while the alkane is not.

Oriented-Films as Rewriteable Anchoring Layers. Once orientational orderhas been imprinted on a thin film using LCI, the film can be used as ananchoring layer to control director alignment in a LC cell. In fact, ifa surface is sufficiently flat, a singly molecular monolayer can controldirector alignment throughout the entire thickness of a LC cell(hundreds of microns). Anchoring layers are a key part of most LCdevices, and therefore a great deal of effort has been expended towarddeveloping improved thin film materials and processing techniques toinduce strong anchoring in a preferred direction. The most widely-usedapproach for planar anchoring is to mechanically rub a polymer film(often polyimide), introducing grooves that align the LC director in aparallel direction. This method has certain disadvantages, includingbuild up of static charge and generation of small wear particles thatmust be removed prior to component assembly. The anchoring induced byrubbing is also permanent, which makes it incompatible for use inLC-based information storage devices and some other applications.Uniformly oriented anchoring layers prepared by the LCI process of thepresent invention can offer advantages over present methods. Theseinclude simplified device fabrication by elimination of steps involvingspin coating, rubbing, curing, and other alignment layer preparationprocedures. The alignment layer could even be formed in situ, afterdevice assembly, by melting and recrystallizing the monolayer in analignment field. The anchoring direction can be altered as needed—withhigh spatial resolution—by local melting and recrystallization. Thus,anchoring films prepared by the LCI process of the invention arerewriteable.

Rewriteable anchoring films have a number of potential uses. If a laseris used to locally melt the film followed by recrystallization under theaction of an external alignment field, the local orientation can bevaried as desired. Patterned films can then be used to control anchoringin a LC cell, with various applications in photonics. These range fromgeneration of complex patterns for use as waveguides, diffractiongratings and holograms to high density information storage devices, towide viewing angle LC displays. Alternatively, patterned films couldserve as templates upon which additional building blocks could bedeposited to form a 3-dimensional material. Few methods have beendeveloped for patterning anchoring films with small-scale features, andnone of the methods is rewriteable. A representative device, a hybridLC-based magneto-optical phase-change memory device, utilizing anoriented material formed in accordance with the present invention isillustrated in FIG. 9. The device in FIG. 9 illustrates how LCI can beused for information storage.

Referring to FIG. 9, device 10 includes laser 2, beamsplitter 4,photodiode 6, external field generator 8, and cell assembly 20. Assembly20 includes sequentially glass layer 22, rubbed polyimide layer 24, andreflective metal film layer 26. Intermediate rubbed polyimide layer 24and reflective metal film layer 26 is LC/solute mixture 28 andorientable film 30. Oriented film 30 lies adjacent metal film 26 withLC/solute mixture 28 intermediate oriented film 30 and rubbed polyimidelayer 24.

Information is read and written as follows. To record a bit, laser poweris increased to locally melt the orientable film. Upon cooling, the filmcrystallizes with an orientation parallel to the LC director above themelted region. When the external field is off, the director isestablished by the rubbing direction of the polyimide layer (see arrow).In this case, the orientable film aligns parallel to the arrow and theanchoring direction is the same at the upper and lower interfaces of thecell. The director is therefore uniformly aligned throughout and thereflected light (read with reduced laser intensity) is strong. When theorientable film crystallizes with the field on, a bit is erased. In thiscase, the magnet aligns the director perpendicular to the orientablefilm and so it crystallizes with a random orientation. Aftercrystallization, anchoring at the orientable film thus differs from thatat the polyimide layer, causing local director distortions. Thesedistortions scatter laser light causing reduced reflected signalintensity.

Monolayer films of polyaromatic hydrocarbons (PAH) and PAHs with polarfunctional groups on atomically-flat Au(111) are viewed as suitablesolutes for this, application (see FIG. 10). Gold is a suitablesubstrate because of its chemical inertness, high optical reflectivity,and excellent thermal conductivity. Thin, optically transparent Au(111)films vacuum deposited on mica can be used so that systems can beexamined with transmission polarizing microscopy.

PAH molecules possess several characteristics making them suitable foruse as rewriteable anchoring films. First, their compact size andconformational rigidity result in high diffusion and crystallizationrates, important for fast write/erase operation. Second, varying thenumber of rings in the central core and the nature and number of polarside groups affects the adsorption energy, offering a way to tailor thefilm's melting point, and to ensure selective adsorption of the solute.Suitable films have melting points between 60-70° C., which is wellabove ambient temperature, but within the nematic range of many LCmixtures and easily achieved with a 5-10 mW focused diode laser. Each ofthese compounds is expected to reversibly physisorb on Au. Third, thesesame structural and chemical variables can also be tuned to optimize theanchoring energy, necessary for strong directional alignment. Finally,some of these compounds are used as dyes in dichroic LC displays and donot suffer from problems arising from chemical incompatibility orreactivity. These films need not possess fully crystalline (i.e.translational) order; all that is required is overall uniaxialorientational order.

Thin films of these compounds can be rapidly screened for their abilityto form oriented monolayers by LCI along with determinations of theirmelting points. The procedure uses a LC optical cell with transparent Aufilms as inner windows. The cell is filled with the LC/solute mixture,placed in a magnetic field, and heated to a temperature above themelting point of the monolayer. Upon cooling a film forms at the Au/LCinterface. To determine whether the film was uniformly aligned by thisprocess, and if so, also its melting temperature, the cell is mounted ona variable-temperature microscope stage and gradually heated toprogressively higher temperatures. During heating a much weaker magneticfield will be applied in a direction differing from that used to preparethe film. If the film was aligned during preparation, the LC directorwill resist reorientation under the action of this weak field, becauseit will be fixed by anchoring in its original direction. If the film wasnot aligned, or if the cell temperature exceeds the film's meltingpoint, surface memory of the original alignment direction will be lost,and the weak field will reorient the LC. For slow heating rates, thisevent occurs abruptly, and can be observed in an optical microscope withcrossed polarizers.

After a group of target compounds has been identified meeting thecriteria for orientability and melting point range, molecular-scale filmstructure can be examined using STM. For film patterning with a focusedlaser, a polycrystalline morphology with small domain size is preferred.To achieve the highest spatial resolution, the average domain sizeshould be smaller than the diffraction-limited laser spot size (˜1 μm²).

Localized changes in film alignment by heating with a focused laserdiode in a strong magnetic field can be determined. These measurementsinvolve determination of the laser power density, heating time, andmagnetic field strength needed to cause local melting and imprinting ofrevised orientational order. To measure minimum feature size produced bythe method of the invention, small (1-5 μm²) surface regions can beirradiated at varying laser intensities for varying lengths of time. Theeffect on local film structure can be measured with STM, and quantifiedby computing a local orientational order parameter, S, based on aregional distribution of domain orientations. For a laser with uniformintensity across a circular illumination zone (the intensity profilewill be separately calibrated), S varies radially, being smallest nearthe perimeter and reaching a maximum at the center. This is becausemolecules recrystallizing near the perimeter are immediately adjacent tounmelted film regions, so the local LC director is oriented by competinginfluences the external magnetic field and anchoring over the unmeltedfilm. A related phenomenon has been reported using ‘molecule corrals’.Analysis of the radial variance of the order parameter profile and itsrelationship to the strength of the magnetic field provides informationuseful for further optimization of the molecular building block.

Hierarchically Structured Materials From Particulate Building Blocks.The method of the invention can be used to prepare materials composed oforiented particles, as well as molecules. Particles can be dispersed inthe LC and caused to precipitate upon the surface of the material withan orientation dictated by the LC fluid. The oriented particles mayrange in size from nanometers to microns. Using this procedure multiplelayers can be laid down, each with the desired orientation, producing afilm of arbitrary thickness. The orientation of particles in successivelayers need not be the same. Examples of the types of particles that canbe oriented include carbon nanotubes, and inorganic and biologicalwhiskers and fibers.

The present invention provides a method for orienting solid crystalsgrown in contact with thermotropic liquid crystals. The advantage of LCalignment media is that orientation can be controlled by an externalfield or other alignment influence. Because alignment arises fromadsorbate-LC fluid interactions, rather than from direct interactionbetween the adsorbate and the field, uniaxial films can be prepared froma variety of different building blocks deposited as layers from a LCsolvent or suspension.

To summarize, the present invention provides oriented materials,including monolayer films of small molecules, formed by deposition of anordered component from a LC solvent in an external influence causingalignment of the LC medium develop macroscopically-uniform orientationalorder, which can be controlled by the field strength and other factors.

While the preferred embodiment of the invention has been illustrated anddescribed, it will be appreciated that various changes can be madetherein without departing from the spirit and scope of the invention.

1. A material produced by the process of depositing an oriented particlefrom an oriented liquid crystal medium onto a substrate.
 2. The materialof claim 1, wherein the material comprises a film.
 3. The material ofclaim 1, wherein the material comprises a uniaxial particulate film. 4.The material of claim 1, wherein the material comprises a monolayerfilm.
 5. A material comprising at least one oriented layer, wherein theoriented layer is formed by depositing an oriented particle from anoriented liquid crystal medium.
 6. The material of claim 5, wherein thematerial comprises a film.
 7. The material of claim 5, wherein thematerial comprises a uniaxial particulate film.
 8. A multilayeredmaterial comprising at least one oriented layer, wherein the orientedlayer is formed by depositing an oriented component from an orientedliquid crystal medium.
 9. The material of claim 8, wherein the componentcomprises an organic molecule.
 10. The material of claim 8, wherein thecomponent comprises a particle.
 11. The material of claim 1, wherein theparticle comprises a carbon nanotube.
 12. The material of claim 1,wherein the particle is at least one of a fiber or a whisker.
 13. Thematerial of claim 1, wherein the particle comprises an inorganicparticle.
 14. The material of claim 1, wherein the particle comprises amicrotubule.
 15. The material of claim 1, wherein the particle comprisesa viral particle.
 16. The material of claim 1, wherein the particlecomprises a nucleic acid polymer.
 17. The material of claim 1, whereinthe particle comprises an organic particle.
 18. An oriented materialhaving at least two layers, comprising: a substrate; a first orientedlayer comprising a first oriented component, wherein the first layer isformed by depositing the first oriented component from a first orientedliquid crystal medium onto the substrate; and a second oriented layercomprising a second oriented component, wherein the second layer isformed by depositing the second oriented component from a secondoriented liquid crystal medium onto the first oriented layer; whereinthe first oriented layer is intermediate the substrate and secondoriented layer.
 19. The material of claim 18, wherein the orientedmaterial comprises a film.
 20. The material of claim 18, wherein atleast one of the first or second oriented components comprises anorganic molecule.
 21. The material of claim 18, wherein at least one ofthe first or second oriented components comprises a particle.
 22. Thematerial of claim 18, wherein the first oriented component has anorientation that is the same as the second oriented component.
 23. Thematerial of claim 18, wherein the first oriented component has anorientation that is different from the second oriented component.
 24. Anoriented material having at least two layers, comprising: a substrate; afirst oriented layer comprising a first oriented component, wherein thefirst layer is formed by depositing the first oriented component from afirst oriented liquid crystal medium onto the substrate; and a secondlayer comprising a second component, wherein the second layer is formedby depositing the second component from a second oriented liquid crystalmedium onto the substrate; wherein the first oriented layer isintermediate the substrate and second layer.
 25. The material of claim24, wherein the second component is randomly oriented.
 26. The materialof claim 24, wherein at least one of the first or second componentscomprises an organic molecule.
 27. The material of claim 24, wherein atleast one of the first or second components comprises a particle.
 28. Anoriented material having at least two layers, comprising: a substrate; afirst layer comprising a first component, wherein the first layer isformed by depositing the first component from a first liquid crystalmedium onto the substrate; and a second oriented layer comprising asecond oriented component, wherein the second layer is formed bydepositing the second oriented component from a second oriented liquidcrystal medium onto the substrate; wherein the first layer isintermediate the substrate and second oriented layer.
 29. The materialof claim 28, wherein the first component is randomly oriented.
 30. Thematerial of claim 28, wherein at least one of the first or secondcomponents comprises an organic molecule.
 31. The material of claim 28,wherein at least one of the first or second components comprises aparticle.
 32. A material having a plurality of layers, at least one ofthe layers being an oriented layer, comprising: a substrate; and aplurality of layers, wherein at least one of the layers is an orientedlayer comprising an oriented component, wherein the oriented layer isformed by depositing the oriented component from an oriented liquidcrystal medium onto the substrate.
 33. The material of claim 32, whereinthe oriented component comprises an organic molecule.
 34. The materialof claim 32, wherein the oriented component comprises a particle.